Devices which offer central services in a distributed system for a number of users, the so-called clients, are referred to as servers, whereby the resources of the respective server can be used more effectively than if each client stored said resources locally and permanently themselves, but only for occasional use.
In a communication network according to the session initiation protocol, the servers then referred to as SIP servers are typically subdivided into three categories;
Proxy servers are located near to the client (Latin: Proximus=next, very near) and forward session initiation protocol notifications on behalf of the client, proxy servers appear transparent for the client, redirect servers which inform the client about the next hop(s), in other words the transmission of a data packet and registrar servers which manage the queries from so-called user agent clients in order to register their current address.
Clients can be telephones, in particular so-called softphones, i.e. personal computers with integrated telephones which can either be operated as user agent clients or as user agent servers and gateways for the purpose of connection control, in particular also as “interpreters” between different transmission formats and communication methods.
According to the prior art the SIP servers can either be configured as stateless SIP servers or as stateful SIP servers.
The stateless SIP servers allow requests transmitted by the client and/or directed thereto to be forwarded in a simple manner according to the SIP protocol, the server itself “forgetting” the process, whilst stateful SIP servers store the processes and actively engage in the communication process, in that they compare incoming notifications for instance in respect of their validity with the current state of a connection and reject them if necessary. Extended functionalities such as conference services for instance can be realized by means of stateful SIP servers.